490 research outputs found
Methods for detection and characterization of signals in noisy data with the Hilbert-Huang Transform
The Hilbert-Huang Transform is a novel, adaptive approach to time series
analysis that does not make assumptions about the data form. Its adaptive,
local character allows the decomposition of non-stationary signals with
hightime-frequency resolution but also renders it susceptible to degradation
from noise. We show that complementing the HHT with techniques such as
zero-phase filtering, kernel density estimation and Fourier analysis allows it
to be used effectively to detect and characterize signals with low signal to
noise ratio.Comment: submitted to PRD, 10 pages, 9 figures in colo
Optical/UV-to-X-Ray Echoes from the Tidal Disruption Flare ASASSN-14li
We carried out the first multi-wavelength (optical/UV and X-ray) photometric
reverberation mapping of a tidal disruption flare (TDF) ASASSN-14li. We find
that its X-ray variations are correlated with and lag the optical/UV
fluctuations by 324 days. Based on the direction and the magnitude of the
X-ray time lag, we rule out X-ray reprocessing and direct emission from a
standard circular thin disk as the dominant source of its optical/UV emission.
The lag magnitude also rules out an AGN disk-driven instability as the origin
of ASASSN-14li and thus strongly supports the tidal disruption picture for this
event and similar objects. We suggest that the majority of the optical/UV
emission likely originates from debris stream self-interactions. Perturbations
at the self-interaction sites produce optical/UV variability and travel down to
the black hole where they modulate the X-rays. The time lag between the
optical/UV and the X-rays variations thus correspond to the time taken by these
fluctuations to travel from the self-interaction site to close to the black
hole. We further discuss these time lags within the context of the three
variants of the self-interaction model. High-cadence monitoring observations of
future TDFs will be sensitive enough to detect these echoes and would allow us
to establish the origin of optical/UV emission in TDFs in general.Comment: Publish in ApJ Letter
The use of thermography on the slaughter-line for the assessment of pork and raw ham quality
The left and right hams of forty heavy pig carcasses were thermographed to evaluate whether surface temperature differences were related to meat and ham quality. Thermal imaging analysis showed no differences in average surface temperature among classes of pH or of L* colour co-ordinate nor among classes of ham defects such as veining and red skin. However, hams with a lower fat cover displayed a significantly warmer average temperature surface. Infrared thermography seems to be a practical and non-invasive method to detect hamswith a fat cover below the minimum requested to be submitted to the dry-curing process
Vitamin B12 and homocysteine levels in blood of dairy cows during subacute ruminal acidosis
Abstract. The aim of this study was to investigate the variations of vitamin B12 and homocysteine in blood of dairy cows during subacute ruminal acidosis (SARA). On 228 subjects ruminal liquid was collected through rumenocentesis technique and rumen pH was immediately measured by a portable pH-meter. On the basis of pH values all cows were classified (bovine class) in Group A (animals with rumen pH>5.7), Group B (animals with rumen pH between 5.6 and 5.7) and Group C (animals with rumen pH<5.6). In relation to the acidosis risk depending on the rumen pH (herd class), the herds were classified in Group 1 (normal herds: less than 33 % cows with rumen pH<5.8), Group 2 (critical herds: more than 33 % cows with rumen pH between 5.5 and 5.8) and Group 3 (acidosis herds: more than 33 % cows with rumen pH<5.5). On blood samples, collected by jugular venipuncture, vitamin B12 and homocysteine were measured by chemiluminescent immunological tests. One-way analysis of variance (ANOVA), followed by Bonferroni test, showed significant differences (P<0.05) for vitamin B12 in bovine class and significant differences (P<0.05) for homocysteine in herd class. The influence of rumen pH values resulted in adequate vitamin B12 and homocysteine levels to meet microbial and cow requirements and fatty acids modifications in dairy cows affected by SARA. Moreover, the increase of vitamin B12 could be due to the presence of analogues which interfere with the transport of the vitamin. These findings provide more information on blood modifications during SARA
X-ray flares from propagation instabilities in long Gamma-Ray Burst jets
We present a numerical simulation of a gamma-ray burst jet from a
long-lasting engine in the core of a 16 solar mass Wolf-Rayet star. The engine
is kept active for 6000 s with a luminosity that decays in time as a power-law
with index -5/3. Even though there is no short time-scale variability in the
injected engine luminosity, we find that the jet's kinetic luminosity outside
the progenitor star is characterized by fluctuations with relatively short time
scale. We analyze the temporal characteristics of those fluctuations and we
find that they are consistent with the properties of observed flares in X-ray
afterglows. The peak to continuum flux ratio of the flares in the simulation is
consistent with some, but not all, the observed flares. We propose that
propagation instabilities, rather than variability in the engine luminosity,
are responsible for the X-ray flares with moderate contrast. Strong flares such
as the one detected in GRB 050502B, instead, cannot be reproduced by this model
and require strong variability in the engine activity.Comment: 6 pages, MNRAS in pres
Search algorithm for a gravitational wave signal in association with Gamma Ray Burst GRB030329 using the LIGO detectors
One of the brightest Gamma Ray Burst ever recorded, GRB030329, occurred
during the second science run of the LIGO detectors. At that time, both
interferometers at the Hanford, WA LIGO site were in lock and acquiring data.
The data collected from the two Hanford detectors was analyzed for the presence
of a gravitational wave signal associated with this GRB. This paper presents a
detailed description of the search algorithm implemented in the current
analysis.Comment: To appear in the Proceedings of 8th Gravitational Wave Data Analysis
Workshop (Milwaukee, WI) (Class. Quantum Grav.
Periodic Variability During the X-ray Decline of 4U 1636-53
We report the onset of a large amplitude, statistically significant
periodicity (~46 d) in the RXTE/ASM data of the prototype X-ray burster 4U
1636-53, the X-ray flux of which has been gradually declining over the last
four years. This behaviour is remarkably similar to that observed in the
neutron star LMXB KS 1731-260, which is a long-term transient. We also report
on an INTEGRAL/IBIS observation of 4U 1636-53 during its decline phase, and
find that the hard X-ray flux (20-100 keV) indicates an apparent
anti-correlation with soft X-rays (2-12 keV). We argue that 4U 1636-53 is
transiting from activity to quiescence, as occurred in KS 1731-260. We also
suggest that the variability during the X-ray decline is the result of an
accretion rate variability related to the X-ray irradiation of the disc.Comment: 6 pages. Accepted by MNRA
Swift BAT Survey of AGN
We present the results of the analysis of the first 9 months of data of the
Swift BAT survey of AGN in the 14-195 keV band. Using archival X-ray data or
follow-up Swift XRT observations, we have identified 129 (103 AGN) of 130
objects detected at |b|> 15 deg and with significance >4.8 sigma. One source
remains unidentified. These same X-ray data have allowed measurement of the
X-ray properties of the objects. We fit a power law to the log N - log S
distribution, and find the slope to be 1.42+/-0.14. Characterizing the
differential luminosity function data as a broken power law, we find a break
luminosity log L_*(erg/s) = 43.85+/-0.26, a low luminosity power law slope
a=0.84^{+0.16}_{-0.22}, and a high luminosity power law slope
b=2.55^{+0.43}_{-0.30}, similar to the values that have been reported based on
INTEGRAL data. We obtain a mean photon index 1.98 in the 14-195 keV band, with
an rms spread of 0.27. Integration of our luminosity function gives a local
volume density of AGN above 10^{41} erg/s of 2.4x10^{-3}/Mpc^3, which is about
10% of the total luminous local galaxy density above M_*=-19.75. We have
obtained X-ray spectra from the literature and from Swift XRT follow-up
observations. These show that the distribution of log n_H is essentially flat
from n_H=10^{20}/cm^{2} to 10^{24}/cm^2, with 50% of the objects having column
densities of less than 10^{22}/cm^{2}. BAT Seyfert galaxies have a median
redshift of 0.03, a maximum log luminosity of 45.1, and approximately half have
log n_H > 22.Comment: 15 pages, 15 figures, 2 tables; to appear in the Astrophysical
Journal, July 10, 2008, v. 68
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